Device for plasma-arc treatment of materials

ABSTRACT

A device for plasma-arc treatment of materials in active media wherein the cathode is made of hafnium which extends the service life of the cathode.

5, by Walter H.

pages 532 534, Reinade of hafnium which References Cited UNITED STATES PATENTS gham.. OTHER REFERENCES iques for Electron Tube plasma-arc treatment of materials 2/1953 Deri 8/1965 Weatherly...........l........

, section entitled Hafnium" on H0lj 17/04 313/231,

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inventors David Grigorievich Bykhovsky Konjushenny pereulok. 1/6, kv. l8; Alexandr Yakovlevich Medvedev, ulitsa Kalyaeva, 3, kv. 4, both of Leni U.S.S.R. Appl. No. 798,348 [22] Filed Feb. 11, 1969 [45] Patented Aug. 3, 1971 Feb. 15, 1968 U.S.S.R. 1,218,055

-ARC TREATMENT OF MATERIALS 3 Claims, 1 Drawing Fig. [52] 11.8. 219/145. 313/231, 313/31 Field of 311,211;219/145,75, 121

United States Patent [32] Priority [54] DEVICE FOR PLASMA DEVICE FOR PLASMA-ARC TREATMENT OF MATERIALS The present invention relates to devices for plasma-arc treatment of materials and more particularly it relates to devices for plasma-arc treatment of materials in active media, preferably for plasma-arc cuttingof metals.

The device can also be employed for surface fusion, for example for the removal of a defective surface layer from castings and cast products or for the correction of such local defects as cracks, pits, scabs.

The invention can also be used for heating chemically active gases and their mixtures and for the thermochemical surface treatment of metal, e.g. nitriding, casehardening and similar processes.

The present invention can be used both with and without additional protection of the cathode assembly.

Known in the art is a device for plasma-arc treatment of materials in active media, preferably for cutting metals, comprising a cathode secured in a cooled cathode holder. The latter is connected by an insulating bushing with a body which If a number of nozzles are used, they are separated by an insulating gasket 12 (shown by dotted lines in the drawing). The coolant may be either water or gas. The body has a hole 13 for the supply of plasma-generating gas, e.g. air, nitrogen, oxygen or mixtures of gases containing nitrogen or oxygen.

Press fitted into the cathode holder 2, flush with it, is the cathode 14 made of a metal forming chemical compounds with the components of the medium. These compounds should have the following properties. They should form a 0 high-melting film of, say, oxides, nitrides or carbides on the accommodates cooled nozzles with central passages intended to stabilize the arc discharge. The cathode is made of a metal producing chemical compounds with the components of the medium. Such metals are zirconium, lanthanum, strontium, yttrium, thorium and their alloys, as seen in US. Pat. No. 3,198,932.

However, the employment of the above-described device is limited by the following disadvantages of the cathode, or the material it is made of.

The above-described cathode has a short service life, in fact not over 4 hours at are currents of 300 A.

At arc currents exceeding 300 A., wear of the cathode becomes so rapid that it becomes practically impossible to utilize such a cathode.

Furthermore, the known cathode is insufficiently stable in operation.

An object of the present invention is .to eliminate the aforesaid disadvantages. I

The main object of the present invention is to provide a device for plasma treatment of materials in active media, preferably for cutting metals, whose cathode will last beyond a minimum value or will practically not wear out at all at are currents of 300 A. The device should give stable service at are currents above 300 A., and the cathode service life should be well over 4 hrs. at-currents exceeding 300 A.

This object is accomplished by providing a'device for plasma-arc treatment of materials in active media, preferably for cutting metals wherein the cathode is made of a metal producing chemical compounds with the components of the medium. According to the invention, such a metal is hafnium (Hf).

Given below is a detailed description of the device by way of example with reference to the accompanying drawing of a preferred embodiment which shows schematically a longitudinal section through the device for plasma-arc cutting of metals in an active medium.

The device for plasma-arc cutting of metals in active media consists of a body I accommodating a cathode holder 2 anda nozzle 3 with a central passage 4 intended to stabilize the are discharge 5.

To prevent the formation of a double are in case the nozzle 3 comes accidentally in contact with the work 6, or to increase the total length of the stabilizing passage 4, there may be installed at least one more nozzle 7 (shown by dotted lines in the drawing). I

The cathode holder 2 is fastened in the body I by means of the insulating bushing 8 and cooled by the coolant supplied through a pipe 9 installed inside the cathode holder 2. The nozzles 3 and 7 have passages 10 and 1! for the supply of coolant.

working surface of the cathode 14. In addition, these compounds would be characterized by high values of thermionic emission, Le. a low work function of electrons. Moreover, these compounds should exhibit high stability under the conditions of repeated heating and cooling.

In accordance with the invention, hafnium was selected for the cathode because, when reacting with the oxygen, nitrogen and carbon contained in the plasma-generating gas, it is covered with a film on the working surface consisting of highmelting oxides, nitrides or carbides of hafnium. This film has a low work function of electrons.

Besides, the structure and the thermal and physical characteristics of the film hamper the further reactive diffusion of the active components of the plasma-generating medium, stopping it altogether in some cases.

This phenomenon makes it possible to considerably prolong the life of the cathode and to increase the working current above 300 A.

The cathode holder 2 of the device is under negative potential while the work 6 is under positive potential. The are discharge 5 burns between the cathode l4 and the work 6, passing through the stabilizing passage 4 of the nozzle 3. If the additional nozzle 7 is used, the discharge 5 also passes through the stabilizing passage 15 of the nozzle 7. In such a case the nozzles 3 and 7 are installed so as to align the axes of the cathode 14 with those ofthe passages 4 and 15.

The active gas enters the device through the hole 13 and passes, together with the arc discharge 5, through the passages 4 and 15.

After the above-described device with a hafnium (Hf) cathode has been tested by cutting low-carbon steels in the air it has been established that the hafnium cathode suffers practically no wear at are currents of up to 400 A. At arc currents exceeding 400 A. the operational life of the cathode amounted to 10-16 hrs.

The use of the service currents of 400 A. and higher has made it possible to increase the cutting speed of low carbon steel l,52 times as compared with the speeds attainable now.

We claim:

1. In a plasma-arc device for treating material in an active medium having a central cathode holder and cathode, an annular passageway between the cathode holder and a housing, plasma-generating gas inlet means to the annular passageway, at least one central nozzle between the cathode and a workpiece, and a power source, the improvement consisting in that I said cathode is made of hafnium, the are discharge between said cathode and the workpiece is stabilized by the plasmagenerating gas passing concentric the cathode axis through the central nozzle and said cathode reacts with the active components in the plasma-generating gas to form a high-melting film on its working surface, which film has a low work function and substantially increases the life of said cathode.

2. A plasma-arc device as claimed in claim 1 wherein said central nozzle is electrically insulated from said cathode and said cathode holder and central nozzle have coolant passages therein.

3. A plasma-arc device as claimed in claim 1 wherein said reactive components in the plasma-generating gas are selected from a group consisting of oxygen, nitrogen or carbon and said high-melting film is formed of a compound selected from the group consisting of oxides, nitrides or carbides of hafnium. 

1. In a plasma-arc device for treating material in an active medium having a central cathode holder and cathode, an annular passageway between the cathode holder and a housing, plasmagenerating gas inlet means to the annular passageway, at least one central nozzle between the cathode and a workpiece, and a power source, the improvement consisting in that said cathode is made of hafnium, the arc discharge between said cathode and the workpiece is stabilized by the plasma-generating gas passing concentric the cathode axis through the central nozzle and said cathode reacts with the active components in the plasmagenerating gas to form a high-melting film on itS working surface, which film has a low work function and substantially increases the life of said cathode.
 2. A plasma-arc device as claimed in claim 1 wherein said central nozzle is electrically insulated from said cathode and said cathode holder and central nozzle have coolant passages therein.
 3. A plasma-arc device as claimed in claim 1 wherein said reactive components in the plasma-generating gas are selected from a group consisting of oxygen, nitrogen or carbon and said high-melting film is formed of a compound selected from the group consisting of oxides, nitrides or carbides of hafnium. 